Method of spray forming destructible forms



A. J. ROBERT 3,538,213

ZSfieets-Sheet 1 INVENTOR.

AIME J. ROBERT 44 ATTORNEY Nov. 3, 1970 METHOD OF SPRAY FORMING DESTRUCTIBLE FORMS Filed Dec.

Nov. 3, 1970 A, J, ROBERT 3,538,213

METHOD OF SPRAY FORMING DESTRUCTIBLE FORMS Filed Dec. 20, 1967 2 Sheets-Sheet 2 INVENTOR. /5 T AIME J. ROBERT I ATTORNEY United States Patent Int. Cl. B29c 1702,- B28b 1/32 US. Cl. 264-225 17 Claims ABSTRACT OF THE DISCLOSURE The process of making readily destructible forms by spraying a slurry of plaster, binder, filler and solvent onto a heated mold surface whereby the plaster particles form, immediately adjacent to the mold surfaces, a predominantly plaster-binder layer backed by a predominantly filler-binder layer. The predominantly plaster-binder layer sets on contact with the heated mold surface so as to assume the contour thereof and together with the fillerbinder backing layer forms a rigid, yet readily destructible, hollow form. An aqueous slurry is used wherein the binder is either polyvinyl alcohol or polyvinyl acetate and the filler is a pulp filler.

This invention relates to destructible forms particularly those used in connection with the building of tanks or containers for gasoline, oil or other liquids, and which are formed of a number of layers of flexible materials. Tanks of this type are, for example, employed as fuel and oil tanks for airplanes and may be of the type which when punctured by a bullet or other means seal the hole made in the wall of the tank.

Tanks of this type, of large size, have heretofore been built about corrugated cardboard shells which are individually prepared by cutting, bending, and pasting of literally hundreds of individual pieces, by hand, into the complex contoured shapes desired. Spaces, between cardboard pieces making up the layers providing the outer surface of thusly built cardboard shells, are frequently filled-in with molding plaster so as to provide a smooth continuous outer surface. Generally, these forms are made in two halves having generally convex outer surfaces so that when placed edge to edge they provide a hollow shell having an exterior surface conforming to the shape of the desired fuel cell. Layers of flexible, preferably elastomeric, material are built up around this outer surface and are cured thereon so as to assume the shape of such outer surface. Thereafter, hot water is introduced into the hollow interior of the shell through one of the openings necessarily provided in the fuel cell, for admission of fuel, oil or other liquids. The hot water disintegrates the cardboard shell so that the latter can be readily washed out of the interior of the fuel cell leaving the fuel cell intact. The just described cardboard molds are, of course, extremely time consuming and expensive to build, particularly when it is recognized that each shell is destroyed after a single use. As noted above, these cardboard shells are handmade, in a form provided for that purpose, by cutting, fitting and gluing hundreds of small cardboard pieces together so as to have the desired shape and of sufficient strength so that the form may be handled while the fuel cell is built thereon.

It will be apparent from the above that even where a number of identical fuel cells are to be manufactured, a cardboard form must be handmade for each such fuel cell.

It is an object of the present invention, therefore, to provide a relatively inexpensive and simple method for making destructible hollow forms.

3,538,213 Patented Nov. 3, 1970 ICC It is a further object of the present invention to provide such a method which will permit destructible forms to be made in a fraction of the time now required.

It is still another object of the present invention to provide a method for making destructible forms having a very smooth surface on which the fuel cells may be built.

It is a concomitant object of the present invention to provide a method for making forms as described, using relatively inexpensive materials and requiring relatively little skill.

A still further object of the present invention is to provide a method as described for making hollow forms having a smooth outer surface backed by a relatively rigid substrate supporting said outer surface for permitting the form to be readily handled, both the surface and the substrate, however, consisting of materials which will permit the form to be readily destroyed by the mere application of hot water.

A further object is to provide an apparatus for carrying out the above method.

According to the present invention, plaster, binder, filler material and a solvent are mixed together to form a slurry. The slurry is then forcefully sprayed onto the surface of a mold where the slurry automatically forms, immediately adjacent the mold surface, a first layer comprising a greater proportion of plaster than of filler material, backed by a second layer comprising a greater proportion of filler material than of plaster. The first layer, which forms at the interface between the sprayed material and the mold surface, hardens substantially upon contact with the mold surface after having assumed substantially the shape and contour of said mold surface. The second, backing layer, forms, when dried, a substantially rigid yet readily destructible support structure for supporting the first layer.

Other objects and advantages of this invention will appear from the following description and claims.

In the accompanying drawings:

FIG. 1 is a perspective view of a mold having a generally concave mold surface used to practice the method of the present invention;

FIG. 2 is a diagrammatic illustration of a nozzle and spray arrangement for carrying out the method of the present invention;

FIG. 3 is a perspective view of a destructible form made according to the method of the present invention, after removal thereof from a mold such as illustrated in FIG. 1;

FIG. 4 is a perspective illustration of a pair of complementary hollow forms made using the method according to the present invention and joined together to provide a hollow mold shell;

FIG. 5 is a partial, transverse, enlarged, sectional view through a portion of the wall of the hollow from illustrated in FIG. 3; and

FIG. 6 is a partly sectional, elevational view of a nozzle for carrying out the present invention.

In the building of fuel or oil tanks of flexible material particularly for use on aircraft, and including those capable of sealing holes made by penetration through the walls of the tanks by bullets or the like, some rigid forms must be employed to maintain the flexible layers of which the tank walls are formed in approximately correct positions.

The construction of tanks or containers of this type is greatly facilitated by the use of building forms which are readily destructible, the material of which the forms are made, being of such a nature that when a solvent such as water, or the like, is added, the form disintegrates and the component parts thereof can be removed through existing inlet or outlet openings in the tanks, without cutting or in any way damaging the same. The destructible forms must have sufficient strength to be able to withstand the uses to which the forms are put during the building of the tanks, for example, such pressures as may be applied to cement adjacent layers or edges of materials together, and to permit the forms to be readily handled, without danger of breaking during the fabricating of the tanks or fuel cells.

It is, of course, desirable to make these breakable building forms as light as possible so that they can be easily manipulated during the construction of the tanks thereon.

These destructible forms are preferably hollow forms such as illustrated in FIG. 3. A complementary pair of such forms may be assembled together edge to edge, as seen in FIG. 4, to form the d'estructible hollow mold shell. Of course, as many forms as desired may be assembled together to form any desired shape.

Referring to FIG. 1, a more or less permanent mold 1, of desired shape, is first constructed. Such a mold is H preferably made of fiber glass reinforced plastic so as to be able to withstand the application of heat during latter stages of the process. Other materials may, of course, be used so long as they are substantially dimensionally stable in the temperature ranges later defined herein, can be readily shaped into desired form, exhibit a smooth mold surface, and have sufiicient rigidity and structural strength for the purposes intended. The permanent mold I, seen in FIG. 1, is preferably in the form of a female mold, i.e., one having a generally concave mold surface 1a.

According to the present invention plaster material, a binder, a filler and a solvent are thoroughly mixed together, for example in a vessel 2, FIG. 2, and thereafter transferred to a pressure vessel 3 which has an upper portion connected to a compressed air source 4, the flow from which can be controlled by means of valve 5. It will be understood that the mixing can alternatively be performed right in a suitably constructed pressure vessel 3 as is within the knowledge of those skilled in the art. A spray nozzle 6 may be held by an operator as shown in FIG. 2. The nozzle 6 is located at the end of a hose 7 which communicates with pressure vessel 3 so that when the valve is opened compressed air in the upper portion of vessel 3 forces the slurry through hose 7 and out of nozzle 6. An air conduit 8 also communicates with the pressure vessel 3 and feeds into the conduit 7 upstream of nozzle 6. A manually operated valve 16 can control the supply of air through conduit 8. In this manner, the stream of slurry which is forced out of nozzle 6 by the compressed air in the pressure vessel 2, is simultaneously dispersed by the stream of compressed air introduced via conduit 8. This stream of compressed air disperses the slurry into the form of a spray thereby separating, at least partially, the plaster from the filler material therein.

In practicing the invention the mold 1 is initially heated to a temperature preferably in the range of approximately 180 F. to 225 F. Then, a wax, or similar parting agent, is applied to the mold surface In. With the nozzle 6 held relatively close to the mold surface In, i.e., preferably a distance of approximately 6 to inches from the surface 1a, the valves 5 and 6a may be opened and the finely dispersed slurry 9 sprayed onto the mold surface 1a. The spray is preferably uniformly played over the surface 1a so as to provide a covering of substantially uniform thickness. I have found that with the temperature ranges defined herein and using a slurry comprising the materials in substantially the proportions set forth herein, and in the manner I have set forth, the spray automatically and simultaneously forms, on the surface In of the mold, a first layer containing a substantially greater proportion of plaster than of filler material, and a second layer, behind said first layer, containing a substantially greater proportion of filler material than of plaster. I have also found that the first layer is relatively thin and contains substantial proportion of plaster. This first layer sets almost immediately upon impact with the mold surface 1a and assumes the shape, i.e., conforms to the contour of, surface 1a. Because of the large proportion of plaster contained therein the first layer displays an extremely smooth outer surface adjacent to, and conforming with surface 1a. The first outer layer thus does not contain any, or at least only very little filler material, and therefore provides a surface layer having the texture and appearance of an exclusively plaster mold surface. The filler material is found in increasing quantities in consecutive second layers which simultaneously form behind the said first, outer, layer.

After spraying an approximately /8 inch thick coat of slurry over the entire surface In of the mold, the sprayed material is permitted to dry. Then, a second coat is sprayed onto the rear of the at least partially dried first coat. It will be remembered that the first coat comprises a first layer substantially free of filler material and a second layer or layers comprising progressively more and more filler material. The second coat is sprayed onto the last of the second layers and is then permitted to at least partially dry. Any desired number of additional coats may thereafter be sprayed behind the second coat, in the manner already described, to give the form the desired wall thickness. When the desired wall thickness is achieved, the mold 1 is placed in an oven for curing of the sprayed material. The binder material, as it sets, binds together not only the plaster particles but also the filler particles as well as binding together the various coats and the various layers within each coat.

FIG. 5 shows a section through the Wall of a form 10 made according to the present invention. The actual thickness of the wall of form 10 is preferably approximately /8 to /2 inch. It will be seen that form 10 comprises a first coat 10a, a second coat 10b, and a third coat 100. Each of these coats, particularly coat 10a, comprises a first layer 11a containing practically no filler material and consisting substantially entirely of plaster, and binder materials. Surface 11a of this first layer gives the appearance of being entirely of plaster, is extremely smooth, and conforms to the contour of mold surface 1a. Behind the first layer 11a is a second layer 12a, also part of the first coat 10a, and which contains substantial amounts of filler material, which latter together with the binder material provides body and rigidity for the form 10. The second and third coats 10b and are similarly constituted. It is, of course, not necessary to use the same amounts of plaster in the second and third coats as used in the first coat since surface smoothness and texture are important only at the surface 11a. In fact, substantially lower proportions of plaster are preferably used in the second and third coats than is used for the first, or outer coat, 10a. These latter coats, it will be noted, are merely added to provide increased body and rigidity to the shell so that the latter will possess the necessary strength to permit fabrication of the tanks or fuel cells thereon.

PERMANENT MOLD Permanent mold 1 can be formed by suitable means well known to those skilled in the art, and therefore requires no further discussion here. Preferably, mold 1 is formed of fiber glass reinforced plastic in the shape of a hollow shell having an inner, generally concave, smooth, mold surface In of the desired shape.

SLURRY COMPOSITION The composition of the slurry 9 for applying the first coat 11a may be generally stated as follows:

Approximately 10 to 12 parts by weight of a binder material which is preferably polyvinyl acetate,

Approximately 1.9 to 2.3 parts by weight of a filler material which is preferably paper pulp,

Approximately 13 to 15 parts by weight of plaster, preferably fresh molding plaster, and

Approximately 20 to 30 parts by weight of a solvent, preferably water.

For the second and following coats, the plaster and water content are preferably reduced in the same proportion as set forth in the hereinbelow given examples.

The important requirements which are met by a proper slurry composition according to the present invention are:

(1) A viscosity which enables it to cling to the mold surface on which it is sprayed yet not so thick that proper spraying and dispersing cannot be achieved;

(2) A suitable working life to permit adequate spraying time before undue drying or setting takes place;

( 3) It must dry without warping.

Another useful binder material is polyvinyl alcohol which has the followin characteristics particularly suitable to the present process:

(1) Polyvinyl alcohol is soluble in water and in alcohol;

(2) Polyvinyl alcohol solution dries to a solid which imparts suitable strength and binder characteristics to the pulp filler as well as to the plaster material;

(3) Polyvinyl alcohol is unaffected by the elevated temperatures required for the curing of the fuel cells or containers;

(4) Polyvinyl alcohol acts as a wetting agent for the pulp.

(5) Polyvinyl alcohol is soluble in water, also after hardening, so that after the PVA has hardened upon being dried, to provide structural rigidity for the hollow mold shell, it may be readily dissolved by the mere application thereto of water. When water, particularly boiling water is supplied to the interior of the destructible shell, the Water rapidly penetrates the entire shell structure and causes it to break down.

SPRAY NOZZLE Spray nozzle 6, shown in FIG. 6, has a mouth 6a at one end, and a hand-grip portion 6b extending substantially at right angles to the direction of spray, and forming a generally L-shaped housing. A curved, generally L- shaped passage 7a is provided for conveying the slurry from hose 7 toward the nozzle mouth 6a. It will be noted that passage 7a has no obstructions and curves gradually toward mouth 6a. Conduit 8 communicates, via passage 8a with an annular chamber 13 which surrounds the downstream end of passage 7a, Chamber 13, in turn, has annularly spaced outlet openings communicating with an annular coically-shaped space 8c, formed between the nozzle 6 proper and the nozzle tip 14. The latter is held in position by a suitable threaded collar 15 and has a central bore through which the slurry and air mixture is discharged in the form of a spray. A suitable spring-loaded trigger 60 operates movable valve member 16, when it is depressed, and simultaneously actuates a switch 60' which is in a suitable circuit with the solenoid valve 5. The conicity of chamber 80 is chosen such that the air from conduit 8 will be moving in generally the same direction as the slurry stream when the two meet.

The air stream supplied through conduit 8 disperses the otherwise solid stream of slurry during spraying and assists in propelling the dispersed stream toward the mold surface 1a.

EXAMPLE The following example will illustrate the method according to the present invention:

(1) A Fiberglas mold 1, having a mold surface area 1a of approximately 7,500 square inches, was subjected for about minutes to a temperature of approximately 20 F.

(2) Immediately upon removal from the oven the mold surface 1a was waxed using a paste wax, which was prewarmed to make it spread easily.

(3) As soon as the waxing was complete the mold was sprayed with the first coat of a slurry which had been previously prepared in the manner and in the proportions 63 substantially as set forth below. A nozzle 6 having a mouth 6a with a central bore approximately 4 inch in diameter, was used with an air pressure source providing air at a pressure of approximately 10 pounds per square inch above atmospheric.

The slurry was prepared in the following manner:

(a) PVA solution: Three parts by weight of polyvinyl alcohol (purchased under the trade name Gelvatol 20/30) was mixed with 6 parts by weight water, and a suificient quantity of denatured alcohol to reduce the viscosity of the thus formed mixture to a viscosity of 42 (seconds drainage time using a Ford cup having a 4 inch diameter opening in its 45 conical bottom) measured at about 70 F. (As an example, a solution of 30 parts by weight Gelvatol 20/30, 60 parts by weight water, and 45 parts by weight denatured alcohol has a viscosity of 70 (seconds drainage time at a temperature of approximately 70 F., using a Ford cup having a inch opening in its 45 conical bottom)). While a viscosity of 42 is preferred, PVA solutions with viscosities in the range of 30 to 60 (measured as above) appear to yield satisfactory results.

(b) Eleven parts by weight of the above PVA solution (a) was mixed with 17 parts by weight of hot water and 2.1 parts by weight of pulp (pureferably hammermilled corrugated cardboard). The mixture was allowed to stand for about 1 hours to assure thorough soaking of the cardboard material.

(0) 11.5 parts by weight of water were mixed with 13.9 parts by weight of white molding plaster (Hammer Brand Molding Plaster, a product of the Canadian Gypsum Co.).

(d) Mixtures (b) and (c) were then thoroughly mixed together as in vessel 2 for about 30 seconds.

(e) The mixture (d) was then transferred to pressure vessel 3.

(f) After the mixing step, step (d), spraying of the first coat must be completed within the usable life of the slurry-found to be approximately 12 minutes.

(g) A second coat was sprayed about 10 minutes after the first coat was completed. And a third coat about 10 minutes after the second coat. The slurry for the second and third coats was essentially the same as that for the first coat just described except that. only half the amount of plaster was used. The amount of water was reduced acordingly so as to maintain the same flow characteristics for the slurry. Thus the mixture for the second and third coats consisted of 11 parts by weight PVA (solution (a)); 2.1 parts by weight, pulp; 7 parts by weight plaster; and 22.8 parts by weight water.

Ten minutes after the second coat, the third coat was sprayed in the same manner as set forth above.

Thereafter, the mold unit was placed in a hot aircirculating oven at 200 F. to dry, for approximately 6 hours.

The hollow form was then removed from the oven and after cooling was ready to be connected to a corresponding form prepared in the same manner, as illustrated in FIG. 4.

While the above slurry composition appears to yield satisfactory results the following slurry composition ranges were also found satisfactory. The preferred quantities within such ranges are separately set forth below:

For step (a) above, a PVA solution having a viscosity in the range of between approximately 38 and 44 seconds, preferably 42 seconds;

For step (b) above, a water content of between approximately 13 and 17.2 parts by weight, preferably approximately 14.8 parts by weight; approximately 11 parts by weight PVA solution, and 2.1 parts by weight of pulp;

For step (0 )above, a water content of between approxi mately 7.2 and 10.0 parts by weight, preferably approximately 9.6 parts by weight; and approximately 13.9 parts by weight plaster;

For step (g) above, a preferred plaster content of approximately 6.95 parts by weight and a water content of between approximately 18.5 and 24.0 parts by weight, preferably approximately 20.0 parts by weight.

The forms 10, of a destructible shell may be secured together in any suitable or desired manner, and suitable attaching members (not shown) may be secured to the forms to facilitate the correct assembly thereof.

After forms have been assembled, as shown in FIG. 4, the construction of the tank or container may be started. The inner layer of material of the tank is first applied to the exterior of the destructible shell in any suitable manner. This layer, may, for example, be a fabric impregnated, or coated, with a synthetic rubber or similar material which is inert with reference to and impermeable by gasoline, oil, or other liquid to be contained in the tank. This fabric may be cut into suitable pieces which may be applied to different sides of the destructible shell, and the adjacent edges of the pieces may be cemented together to form a complete inner layer, or lining, of the container. The fittings to be installed on the container, or fuel cell, are also secured to the inner layer by cementing or otherwise. It will, of course, be understood that the outer surface of the destructible shell may be coated with talc or any other material for preventing any of the layers of the tank, or the cement employed in seaming together different portions of the tank, from adhering securely to the destructible shell. After the inner layer of material of the tank or container has been applied, the remaining layers of material may be applied to such inner layer in any suitable or desired manner. These various layers may be cemented together in any suitable, Well known, manner so as to form a substantially unitary wall of the tank or container.

When the tank has been completely built on the shell 10, 20, the shell is destroyed by supplying hot water, or other suitable solvent, to the interior thereof, through one of the openings of the tank or container. The materials of the destructible shell are either soluble in, or at least softened by, the application of such water or solvent. The dissolved, or softened, shell materials in the interior of the tank or container may then be conveniently flushed out by additional water or solvent supplied through ducts passed into the interior of the tank through the various tank openings.

Destructible mold shells of the type described have the advantage that they are inexpensive to produce, and after a tank or fuel cell is completed the shell can be readily destroyed e.g., by dissolving, or at least softening, the material thereof so that it can be readily removed through the inlet or outlet openings of the fuel cell. The materials for making the destructible shell are relatively inexpensive and the spraying time required is relatively short. Furthermore, by being to a large extent soluble in water, the hollow shells can be quickly dissolved and easily removed from the interior of the finished containers without risking damage to the latter.

As previously noted the forms built according to the present invention are light in weight and readily disintegrate when soaked with water.

While there has been described what is at present considered to be the preferred method and apparatus, according to this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, intended to cover all such changes and modifications that fall within the true spirit and scope of the invention.

Having thus described my invention, what I claim and desire to protect by Letters Patent is:

1. In a method of making a destructible hollow form having a surface conforming substantially to the surface of a mold, the steps of:

preparing an aqueous slurry of plaster, binder selected from the group consisting of polyvinyl alcohol and.

polyvinyl acetate, and pulp filler; heating said mold;

directing a stream of said slurry toward said mold surface;

simultaneously injecting a gaseous medium under pressure into said stream for dispersing the latter into a spray in which the filler material is substantially separated from the plaster and for propelling said spray onto said mold surface so that said spray will form on said mold surface a first layer comprising a greater proportion of plaster than filler and simultaneously therewith will form a second layer behind said first layer, said second layer comprising a greater proportion of filler than plaster, whereby when said layers set said first layer displays an outer relatively smooth surface conforming substantially to said mold surface and said second layer provides a support for said first layer.

2. The method according to claim 1, wherein said binder comprises polyvinyl alcohol.

3. The method according to claim 2, wherein said polyvinyl alcohol has a viscosity as measured by the Ford cup method in the range of 30 to 60 at a temperature of approximately 70 F.

4. The method according to claim 1, wherein said binder is polyvinyl alcohol, said filler is paper pulp, and said plaster is powdered molding plaster.

5. The method according to claim 1, wherein said slurry consists of approximately 10-12 parts by weight polyvinyl alcohol, 20-30 parts by weight water, 1.9-2.3 parts by weight paper pulp, and 13-15 parts by weight plaster, further comprising thoroughly mixing said materials to form said slurry.

6. The method according to claim 1, wherein said slurry consists of approximately 11 parts by weight polyvinyl alcohol solution, approximately 24.4 parts by weight water, approximately 2.1 parts by weight paper pulp constituting said filler, and approximately 13.9 parts by weight plaster, further comprising thoroughly mixing said materials to form said slurry.

7. The method according to claim 1, wherein said binder is polyvinyl acetate.

8. The method according to claim 1, further comprising aplying a parting material to said mold surface to spraying said slurry thereon.

9. The method according to claim 1, comprising heating said mold to a temperature in the range of approximately 180 F. to 225 F. prior to spraying of said slurry.

10. The method according to claim 1, comprising further heating said mold after said spraying step to facilitate setting of the sprayed materials.

11. The method according to claim 10, wherein said further heating step comprises heating said first and second layers on said mold, whereby said binder sets, binding together the plaster of said first layer, the filler material of said second layer, and said first and second layers so as to provide a substantially rigid form.

12. The method according to claim 11, wherein said binder is a solution which dries to a solid when said layers are heated, so as to provide structural strength to said form.

13. The method according to claim 12, wherein said binder in solid form is soluble in water.

14. The method according to claim 1, wherein said slurry is sprayed through a nozzle held close to but spaced from said mold surface and substantially uniformly played thereover.

15. The method according to claim 1 further comprising repeating each of said steps at least once so as to form at least one additional first layer and at least one additional second layer on the back of said first mentioned second layer.

16. The method accordng to claim 15, wherein said mold surface is generally concave, further comprising the steps of forming a second hollow form in the same manner as said first mentioned hollow form; placing said forms together edge to edge so as to provide a substantial- 1y fully enclosed hollow shell whose outer surface corresponds substantially to said generally concave surface of said molds adapted to be covered with a layer of elastomeric material and adapted to have said layer of elastomeric material cured thereon so as to assume the shape thereof; and thereafter filling the interior of said hollow shell with hot water so as to destroy said shell at the interior of the shaped elastomeric material.

17. The method according to claim 1 wherein said first layer constitutes the outer layer of said form and is relatively thin, said second layer constituting an inner support structure for said first layer and being substantially thicker than said first layer, said relatively thin first layer and said thicker second layer forming automatically and simultaneously in response to said spray impinging on said mold surface.

References Cited UNITED STATES PATENTS 2,806,277 9/1957 Hand et al. 264-309 Sucetti 106-111 Sefton 260-296 Heijrner 260-296 Nagel et al 164-20 Bradley 164-20 Howard 164-21 Kugler 164-19 Hockin 164-25 Lirones 164-24 Great Britain.

JULIUS FROME, Primary Examiner 15 H. MINTZ, Assistant Examiner US. Cl. X.R. 

